Ultrasound transducer

ABSTRACT

Acoustic imaging systems are provided. A preferred system includes a protective cover configured to mate with a transducer body. The transducer includes a two-dimensional transducer element matrix array formed by a plurality of individually controllable transducer elements. The protective cover is superposed above the two-dimensional matrix array and is transparent to incident acoustic energy. Preferably, the protective cover is shaped to reduce patient discomfort and repetitive motion injuries to sonographers. Alternative embodiments comprise a shaped two-dimensional transducer element matrix array. Methods for improved ultrasound imaging are also provided.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to copending U.S. provisionalapplication entitled, “IMPROVED ULTRASOUND TRANSDUCER” having Serial No.60/301,282, filed Jun. 27, 2001, which is entirely incorporated hereinby reference.

TECHNICAL FIELD

[0002] The present application generally relates to acoustic imaging.More particularly, the application relates to ultrasonic imaging systemsand methods that use transducers with two-dimensional transducer elementarrays.

BACKGROUND

[0003] Ultrasound imaging systems have become an important diagnostictool in many medical specialties. One important advantage of anultrasound imaging system is real-time scanning. For example, anultrasound imaging system can produce images so rapidly that asonographer can scan internal organs or can discern motion within abody, such as blood flow, with real-time, interactive, visual feedback.This allows the sonographer to examine structures of interest and tomodify the examination in real-time, thereby improving both diagnosticquality and patient throughput.

[0004] Along with the advantages of real-time, interactive, visualfeedback, sonographers are still concerned with system resolution. In anultrasound imaging system, system resolution depends on the system'sability to focus. The ability to focus depends, in turn, on theeffective aperture of a transducer element array in a probe associatedwith the ultrasound imaging system. Currently two types of arrangementsof transducer array elements are used for real-time, ultrasound imagingsystems.

[0005] One arrangement comprises a single transducer element or anannular array of transducer elements. Ultrasound imaging systems usingthis arrangement of transducer array elements rely on mechanical motionof the probe to sweep an acoustic beam over a region of interest.

[0006] A second arrangement of transducer array elements comprises anarray of transducer elements which is activated by electronic circuitswhich produce electronically induced time delays in the transducerelement acoustic outputs. These time delays induce measurable phasedelays, which cause the acoustic beam produced by the transducer elementarray to be steered and/or focused.

[0007] Links between electronic circuits which generate transmit pulsesfor transducer array elements and the transducer array elements thatreceive the transmit pulses are referred to as beamformer channels.Electronic steering and/or focusing of an acoustic beam produced by thetransducer element array is achieved by electronically delaying transmitpulses, on a beamformer channel-by-beamformer channel basis, to createan effective protective cover having varying thickness.

[0008] Due to limits on: (a) the size and complexity of a cableconnecting the ultrasound probe with the processing system and (b) thenumber of beamformer channels available in a reasonably pricedultrasound system, electronic focusing has been limited to a lateraldirection (a direction parallel to the imaging plane). Focusing in anelevation direction (a direction perpendicular to the imaging plane) hasbeen accomplished by placing a mechanical lens, of fixed curvature, onthe probe face.

[0009] Conventional modifications in elevation focusing have beenaccomplished by changing the probe aperture and/or the properties of themechanical lens. Although it is known that changing frequency can changefocal depth (higher frequencies producing deeper focusing than lowerfrequencies), it is not considered advantageous to change frequency tochange focal depth because higher frequencies are attenuated morerapidly in tissue than lower frequencies.

[0010] Consequently, it is known that in order to change elevationfocusing of a transducer element array, one ought to change theelevation aperture and/or change the effective curvature of a lensassociated with the transducer element array. For example, in imaging adeep organ, the lens ought to have a large aperture and mild curvatureand, in imaging a shallower object, the lens ought to have a smalleraperture and a tighter curvature.

[0011] As is known, transducer array elements in an ultrasound probe canbe arranged in a one-dimensional (1-D) array, aone-and-a-half-dimensional (1.5-D) array, or a two-dimensional (2-D)array (the size of a typical 1-D transducer array element is on theorder of 0.5 wavelengths in the lateral direction and is on the order of50 wavelengths in the elevation direction). In a 1-D array, transducerelements are generally disposed in the lateral direction, with a singlerow of elements in the elevation direction. Conventional phase lineararrays and curved arrays are generally considered 1-D transducer elementarrays.

[0012] In a 1.5-D array, transducer elements are mounted in both thelateral and elevation directions, but control and data electricalconnections are symmetrically connected about the elevation center sothat an acoustic beam produced by a 1.5-D array can only be steered inthe lateral direction. In a 2-D array, transducer elements are arrangedin both the lateral and elevation directions, with electricalconnections providing both transmit/receive control and excitationsignals to transducer elements arranged in both directions. An acousticbeam produced by a 2-D array can be steered and focused in twodimensions. An example of a 2-D array ultrasound probe can be found inU.S. Pat. No. 5,186,175.

[0013] The advantages of 2-D array imaging are well known. For example,such advantages include the ability to electronically steer in two (2)dimensions (i.e., both lateral and elevation), enhanced resolution dueto improved elevation focusing, and improved phase aberration correctionthrough refined comparison of propagation velocities. The flexibilityand enhanced resolution associated with 2-D transducers has eliminatedthe need for an acoustic lens shaped to mechanically focus the acousticbeams. However, the transducer elements still need to be protected.Consequently, the faces of 2-D transducers are configured with arelatively flat acoustically transparent material layer.

[0014] Sonographers can obtain images of a region within a body byproperly positioning an ultrasound transducer against the body. In orderto obtain images having diagnostic value, the sonographer may have tomanipulate the position of the probe by sliding, rotating, and/ortilting the probe with respect to the patient.

[0015] A flat transducer face, such as those used with 2-D transducers,degrades image quality because it provides poorer contact with the bodystructures of a patient than a transducer with a curved surface. Morespecifically, a flat transducer surface causes spurious reflections andblock portions of the acoustic aperture. Another disadvantage associatedwith a transducer configured with a flat face is that such transducerseither have sharp edges, which can cause patient discomfort, or thetransducers have an overly broad footprint to permit rounder edges.

[0016] Transducers configured with an overly broad footprint furtherimpair contact between the transducer face and the patient, which cancause a sonographer to apply greater pressure along the longitudinalaxis of the transducer in an attempt to improve contact between thetransducer face and the patient. The increase in sonographer inducedpressure can result in patient discomfort, as well as repetitive motioninjuries to the sonographer. One area where maintaining appropriatecontact between the transducer face and the patient is particularlyproblematic is intercostal cardiac and thoracic imaging. Generally, forthese applications, the transducer housing contains a 2-D array oftransducer elements selected for the expected enhanced resolution due toimproved elevation focusing.

[0017] Consequently, there is a need for an improved transducer thataddresses these and/or other shortcomings associated with conventionaltransducers.

SUMMARY

[0018] Embodiments of the improved ultrasound transducer may beconstrued as providing acoustic imaging systems. In a preferredembodiment, the system includes a shaped protective cover configured tomate with a transducer body. The protective cover is formed, at leastpartially, of a material, which exhibits acoustic properties thatredirect ultrasound energy passing through the material into the body tobe imaged. The shaped protective cover provides patient comfort, anincreased acoustical window, and reduces the incidence of repetitivemotion injuries to sonographers. An ultrasound imaging system configuredwith the improved transducer electronically focuses acoustic energy thattraverses the protective cover.

[0019] Other embodiments of the present invention may be construed asproviding methods for acoustically imaging a patient, for example. Apreferred method includes the steps of: (1) providing a transducerhaving a shaped protective cover formed, at least partially, of anacoustic focusing material; (2) propagating acoustic waves from theprotective cover; (3) receiving acoustic waves reflected from structureswithin the body to be imaged; (4) converting said received acousticwaves to electrical signals; and (5) processing said electrical signalsto produce an image.

[0020] Other systems, methods, and features, of the improved ultrasoundtransducer will be apparent to one skilled in the art upon examinationof the following drawings and detailed description. It is intended thatall such additional systems, methods, and features are included withinthis description, are within the scope of the improved ultrasoundtransducer, and protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The improved ultrasound transducer can be better understood withreference to the following drawings. The components in the drawings arenot necessarily to scale; emphasis instead is placed upon clearlyillustrating the principles of the transducer. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

[0022]FIG. 1 is a schematic diagram depicting a conventional 1-Dtransducer transmitting acoustic energy into a representative body.

[0023]FIG. 2 is a schematic diagram depicting the improved ultrasoundtransducer in association with an image processing system.

[0024]FIG. 3 is a schematic diagram of the improved ultrasoundtransducer of FIG. 2 showing detail of the image processing system.

[0025]FIG. 4 is a schematic diagram illustrating control of thetransducer elements of the improved ultrasound transducer of FIG. 2.

[0026]FIG. 5A is a partial cross-sectional side view illustrating anembodiment of the improved ultrasound transducer of FIG. 2.

[0027]FIG. 5B is a side view illustrating another embodiment of theimproved ultrasound transducer of FIG. 2.

[0028]FIG. 6 is a flowchart depicting preferred functionality of theimaging system of FIG. 2.

[0029]FIG. 7A is a plan view of an alternative embodiment of the presentinvention.

[0030]FIG. 7B is a schematic diagram depicting detail of the protectivecover of FIG. 7A.

[0031]FIG. 8A is a plan view of an alternative embodiment of theimproved ultrasound transducer.

[0032]FIG. 8B is a schematic diagram depicting detail of the protectivecover of FIG. 8A.

[0033]FIG. 9 is a schematic diagram depicting representative placementof the improved ultrasound transducer during a representative thoracicimaging procedure.

[0034]FIG. 10 is a schematic diagram illustrating an acoustic beam inboth transmit and receive modes used to image a target.

[0035]FIG. 11 is a schematic diagram illustrating multiple acousticbeams in both transmit and receive modes used to image multiple targets.

[0036]FIG. 12 is a schematic diagram illustrating the spatialrelationship between the protective cover of the improved transducer ofFIG. 2 and the two-dimensional transducer element array.

DETAILED DESCRIPTION

[0037] Conventional one-dimensional (1-D) phased array transducersutilized for ultrasonic imaging typically incorporate lenses that focusacoustic beams transmitted from the transducers. In particular, themechanical configuration of such a lens typically is selected to focusan acoustic beam from a transducer in an elevation dimension. Theelevation dimension also may be focused mechanically, such as byimplementing a concave shape at the array of the transducer. The lateraldimension typically is focused electronically.

[0038] By way of example, a conventional 1-D phased array transduceruses a lens that promotes focusing of transmitted acoustic energy withina body, e.g., a human body. Oftentimes, the material of such a lenspossesses an acoustic velocity that is less than that of the human body(approximately 1.5 mm/μsec). So provided, the acoustic energy propagatedinto the body by the ultrasound transducer through the acoustic lenstends to converge or focus within the body. Focusing of acoustic energytransmitted from a conventional 1-D transducer within a body is depictedschematically in FIG. 1.

[0039] In FIG. 1, representative acoustic waves 12, 14, 16, 18, and 20are shown being transmitted from transducer 22 via a focusing lens 24.As depicted therein, the acoustic waves tend to focus as they propagatedeeper into body 30 due, at least in part, to the material of the lens24.

[0040] As is known, acoustic energy propagates at various velocities andwith various wave-front shapes depending upon, for example, the acousticvelocity and acoustic impedance of a material(s) through which theacoustic energy is propagated. For instance, the closer the acousticvelocity of a lens material is to that of the body, the closer theenergy is transmitted from a transducer and into the body at theincident angle. Additionally, the closer the acoustic impedance of thelens material is to that of the body, more ultrasonic energy istransmitted from the transducer and into the body.

[0041] As shown in FIG. 2, a preferred embodiment 200 of the imagingsystem incorporates a transducer probe (“transducer”) 202. By way ofexample, transducer 202 can be a two-dimensional (2-D) phased arraytransducer. Transducer 202 is electrically coupled with an imageprocessing system 204. Image processing system 204 provides varioussignals to transducer 202 so as to enable the transducer 202 to transmitacoustic energy via a plurality of transducer elements arranged in a 2-Darray about a transducer face 207. The transmitted acoustic energy aswell as reflected acoustic echoes may then traverse a protective cover206 manufactured from an acoustically transparent material. Thetransducer 202 converts the reflected acoustic echoes into electricalsignals that are returned to the image processing system.

[0042] Protective cover 206 is maintained in position relative to thetransducer body 208 by a nose portion 210 of the transducer body 208. Inparticular, protective cover 206 is adapted to seat at least partiallywithin an aperture (not shown) defined by the nose portion 210. Variousother configurations, however, can be used.

[0043] In prior art transducers, protective cover 206 is configured asan acoustically non-focusing lens. More specifically, protective cover206 is formed of selected material(s) and/or exhibits a particular shapethat enables acoustic energy to be propagated into a body, e.g., a humanbody, without substantially mechanically focusing the acoustic energy.By way of example, prior art embodiments of the ultrasound transducer200 may include a protective cover 206 that is at least partially formedof an acoustic-matching material. Such an acoustic-matching materialpreferably exhibits an acoustic velocity and impedance thatsubstantially match the acoustic velocity and acoustic impedance of atypical body.

[0044] In alternative prior art embodiments, non-focusing is achieved bymaking the transducer face 207 flat or convexly curved and maintaining auniform thickness in that part of the protective cover 206 that lies inthe acoustic path. For instance, a material exhibiting an acousticvelocity within the range of approximately 1.4 mm/μsec to approximately1.6 mm/μsec could be considered an acoustic-matching material formedical diagnostic applications. An acoustic-matching material alsopreferably exhibits an acoustic impedance within the range ofapproximately 1.3MRayl to approximately 1.7MRayl.

[0045] In some embodiments, the acoustically non-focusing protectivecover 206 may be formed of butadiene, styrene butadiene, and/or anassociated classes of rubbers and/or polymers, among others. Thesematerials typically attenuate acoustic energy at approximately 3 dB/cmat 2 MHz and approximately 8 dB/cm at 5 MHz. As is known, conventionallens materials, such as silicone, attenuate acoustic energy atapproximately 9 dB/cm at 2 MHz and approximately 33 dB/cm at 5 MHz.

[0046] It should be noted that one of ordinary skill in the art maychoose to provide a protective cover 206 formed of materials that,individually, may not be considered acoustic-matching materials.However, providing a combination of materials that together exhibitacoustic-matching properties, e.g., an acoustic velocity within therange of approximately 1.4 mm/μsec to approximately 1.6 mm/μsec and anacoustic impedance within the range of approximately 1.3MRayl toapproximately 1.7MRayl, is considered for the improved ultrasoundtransducer.

[0047] By providing an acoustically non-focusing protective cover 206,imaging system 200 may enable transmission of acoustic energy into apatient's body that is suitable for electronic focusing in both thelateral and elevational dimensions. In particular, the imaging system200 may provide acoustic beams that are conducive to comparativelysensitive electronic focusing. This could facilitate improved zoomimaging functionality as compared to other ultrasound imaging systems,which use mechanically focused lenses. It also is presumed that animaging system using an acoustically non-focusing protective cover 206may provide acoustic beams that are particularly well suited forcontrast imaging applications. As described in detail hereinafter,improved imaging systems can include various shapes of protective covers206, which are at least partially formed of acoustic-matching material.

[0048] A disadvantage of the prior art is that the use of a non-focusingprotective cover 206 may be undesirable. A suitable acoustic-matchingmaterial that meets other transducer requirements such as durability,chemical resistance, and biocompatibility may not be available or mayrequire excessive development effort. Furthermore, requirements formaintaining contact between the transducer 202 and the patient maydictate a shape for the protective cover surface that causes substantialfocusing of the acoustic energy. The improved ultrasound transducer 202advances the art of ultrasound imaging by electronically compensatingfor the focusing characteristics of the protective cover 206.

[0049] Referring now to FIG. 3, a preferred embodiment of the imagingprocessing system 204, will be described in detail. It will beappreciated that FIG. 3 does not necessarily illustrate every componentof the preferred system, emphasis instead being placed upon thecomponents most relevant to the systems and/or methods disclosed herein.

[0050] As depicted in FIG. 3, image processing system 204 includes theimproved transducer 202, which is electrically connected to a T/R switch302 of the image processing system 204. T/R switch 302 places thetransducer 202 in either a transmit or receive mode. In order tofacilitate transmission of acoustic energy via the transducer 202 duringoperation in the transmit mode, image processing system 204 includes atransmit beamformer 304 that sets the transmit frequency, ƒ_(o), andmagnitude of various transmit signals. The transmit beamformer 304 is incommunication with a transmit waveform modulator 306 that generates thevarious transmitted signal lines. As illustrated in FIG. 3, the transmitbeamformer 304 and transmit waveform modulator 306 operate under controlof a central controller 310.

[0051] In order to facilitate reception of acoustic energy via thetransducer 202 during operation in the receive mode, the imageprocessing system 204 includes an A/D converter 312, which convertsanalog signals received from the transducer 202 into digital signals. Adigital filter 314, e.g., an RF filter, filters signals outside adesired receive band from the received data. Next, a receive beamformer316 receives the filtered digital signals representing the receivedultrasound echoes.

[0052] The receive beamformer 316 may be designed to receive multipledigital echo waveforms (corresponding with a plurality of sets oftransducer elements from the 2-D array of transducer elements) from theA/D converter 314. The receive beamformer 316 may combine the multipledigitized echo waveforms to form a single acoustic line. To accomplishthis task, a plurality of parallel processing channels within thereceive beamformer 316 may delay the separate echo waveforms bydifferent amounts of time and then may add the delayed waveformstogether, in order to create a composite digital acoustic line.Furthermore, the receive beamformer 316 may receive a series of datacollections for separate acoustic lines in succession over time andprocess the data in a pipeline-processing manner.

[0053] An image processor 318 may contain a suitable species of randomaccess memory (RAM) and may be configured to receive a series ofcomposite digital acoustic lines from the receive beamformer 316. Theacoustic lines can be defined within a three-dimensional coordinatespace. The image processor 318 may be configured to mathematicallymanipulate image information within the received and filtered digitalacoustic lines. In addition, the image processor 318 may be configuredto accumulate acoustic lines of data over time for signal manipulation.In this regard, the image processor 318 may further include a scanconverter to convert the data as stored in the RAM in order to producepixels for display. Each scan converter may process the data in the RAMonce an entire data frame (i.e., a set of all acoustic lines in a singleview, or image/picture to be displayed) has been accumulated by the RAM.

[0054] For example, if the received data is stored in RAM using polarcoordinates to define the relative location of the echo information, thescan converter may convert the polar coordinate data into rectangular(orthogonal) data capable of being raster scanned via a raster scancapable processor. The ultrasound imaging system 204, having completedthe receiving, echo recovery, and image processing functions, to form aplurality of image frames associated with the plurality of ultrasoundimage planes, may forward the echo image data information to a videoprocessor 320 as illustrated in FIG. 3.

[0055] Video processor 320 may be designed to receive the echo imagedata information and may be configured to raster scan the imageinformation. The video processor 320 produces picture elements (i.e.,pixels) that may be forwarded to the display device 322. In addition,the picture elements may be forwarded to a video memory device (notshown). Video memory devices may include a digital videodisc (DVD)player/recorder, a compact disc (CD) player/recorder, a video cassetterecorder (VCR), or other video information storage device. As is knownin the art, these video memory devices permit viewing and or post datacollection image processing by a user/operator in other than real-time.

[0056] As further illustrated in FIG. 3, display device 322 may beconfigured to receive the picture elements (i.e., the pixel data) fromthe video processor 320 and drive a suitable display screen or otherimaging device (e.g., a printer/plotter) for viewing of the ultrasoundimages.

[0057] Many variations of the image processing system 204 presented inFIG. 3 may be operational with the improved ultrasound transducer 202.For example, the receive beamformer 316 may be split into two parts, ananalog portion being between the T/R switch 302 and the AID converter312 (not shown) and a digital portion disposed after the digital filter314 as illustrated in FIG. 3.

[0058] Reference is now directed to FIG. 4, which presents a schematicview illustrating a transducer control system 400. The transducercontrol system 400 controls a two-dimensional transducer element array402. The two-dimensional transducer element array 402 includes aplurality of ultrasonic transducer elements, exemplar ones of which areillustrated using reference numerals 408, 412 and 414. The ultrasonictransducer elements 408, 412 and 414 are arranged in rows and columns,exemplar ones of which are illustrated using reference numerals 404 and406, respectively. Such a configuration is sometimes referred to as amatrix array. However, other transducer element configurations arepossible.

[0059] Although the schematic of FIG. 4, illustrates a planar 8×14 arrayof ultrasonic transducer elements, it should be mentioned that theconcepts of the invention are applicable to any two-dimensionalultrasonic transducer element array configuration, includingconfigurations in which ultrasonic transducer elements are curved in oneor both of the two dimensions. For example, two-dimensional transducerelement arrays having cylindrical, spherical, toroidal, or other curvedsurfaces are possible and may use slightly modified beamforming thanthat associated with the planar two-dimensional transducer element array402 illustrated in FIG. 4.

[0060] Each of the elements 408, 412 and 414 of the two-dimensionaltransducer element array 400 is individually controllable. Specifically,each of the transducer elements 408, 412 and 414 can function as atransmit element and as a receive element, and each receivesindividualized control signals. For example, ultrasonic transducerelement 408 is electrically coupled via connection 416 to atransmit/receive (T/R) switch 418. The T/R switch 418 is controlled by asignal (not shown) from the central controller 310 to permit thetransducer element 408 to function in a transmit mode and in a receivemode.

[0061] When the transducer element 408 is used in a transmit mode, thetransducer element 408 receives a transmit pulse from the transmitbeamformer 304 through connection 426 and via the variable amplifier 422via connection 424. The variable amplifier 422 is used to define thecharacteristics of the transmit pulse applied to the transducer element408 and is controlled by amplitude controller 420 via connection 430.Although omitted for simplicity, each element in the two-dimensionaltransducer element array 402 includes a similarly controlled variableamplifier.

[0062] When the transducer element 408 is used in a receive mode,ultrasonic energy that impinges upon the surface of the transducerelement 408 is converted to an electrical signal. The electrical signalis communicated via connection 416, through T/R switch 418 (which is nowconnected to connection 444 by operation of a control signal from thecentral controller 310) so that the receive signal is applied tovariable gain amplifier 446. The variable gain amplifier 446 amplifiesthe electrical receive signal and supplies the signal over connection448 to delay element 484.

[0063] In a similar manner, the transducer element 412 receives atransmit pulse via connection 436 and supplies a receive signal viaconnection 438 to variable gain amplifier 442. Variable gain amplifier442 supplies the received signal via connection 458 to delay element482. Similarly, transducer element 414 receives a transmit signal viaconnection 458, through switch 456 and connection 454, while the receivesignal is passed via connection 454, through switch 456 and connection462 to variable gain amplifier 464. The variable gain amplifier 464supplies the amplified receive signal on connection 466 to the delayelement 478. Each element in the two-dimensional transducer elementarray 402 is thus controlled, thereby allowing full control over eachelement in the two-dimensional transducer element array 402.

[0064] The variable gain amplifiers 462, 442 and 446, and the delayelements 478, 482 and 484 are all contained within receive beamformer316. While shown as having only three variable gain amplifiers and threedelay elements, the receive beamformer 316 includes sufficientamplifiers and delay element circuitry (and other processing circuitry)for each of the transducer elements in the two-dimensional transducerelement array 402. Furthermore, various multiplexing, sub-beamforming,and other signal processing techniques can be performed by the receivebeamformer 316. However, for simplicity of illustration, the receivebeamformer 316 in FIG. 2 includes only three delay elements. Each of theamplifiers in the receive beamformer 316 is controlled by a signal viaconnection 480 from the central controller 310. The signal on connection480 determines the receive gain applied by each of the variable gainamplifiers 464, 442 and 446. The gain applied by each of the amplifiersmay vary. Similarly, each delay element 478, 482 and 484 is operated bya signal from the central controller 310 via connection 474. Thiscontrol signal determines the amount of delay that each of the delayelements 478, 482, and 484 applies to their respective receive signals.In this manner, the receive aperture can be controlled with a highdegree of precision, because each transducer element in thetwo-dimensional transducer element array 402 includes a respectivevariable gain amplifier 442, 446 and 464 and control circuitry.

[0065] The output of delay elements 478, 482, and 484 are respectivelysupplied via connections 486, 488 and 492 to summing element 494.Summing element 494 combines the output of each delay element andsupplies a beamformed signal on connection 496 to additional processingelements, such as the image processor 318 (not shown). In alternativeconfigurations, the variable gain amplifiers 464, 442 and 446 may belocated after the delay elements 478, 482, and 484, respectively.Further, the outputs of the delay elements 478, 482, and 484 may becombined into sub-arrays and variable gains may be applied to eachsub-array either before or after the sub-array signal passes through itsrespective delay prior to the summing element 494.

[0066] Importantly, the two-dimensional transducer element array 402having individually controllable transducer elements 408, 412, and 414makes the emitted ultrasound pulse pattern variable in two-dimensions.Specifically, the two-dimensional transducer element array 402 can becontrolled with respect to the position of each element within thearray. By having complete control over the entire aperture, the 2-Dtransducer element array control system 400 allows the beam plot of theaperture to be controlled with a high degree of precision.

[0067] Calculation of the delays used in the transmit beamformer 304 andreceive beamformer 316 may be understood with reference to FIG. 10wherein it is desired to focus the image at a target 1002, which may be,for example, some structure within the body to be imaged 30. In thiscase, the transmitted acoustic energy from two-dimensional transducerelement array 402 is brought to a focus at target 1002 and the receivebeamformer 316 focuses the received acoustic energy to maximize thereceive sensitivity at target 1002. In order to transmit acoustic energyfocused on target 1002, the central controller 310 may provide delaycontrol signals to the transmit beamformer 304 for each element of thetwo-dimensional transducer element array 402 via connection 468 and asynchronization signal to provide a time reference for the delays. Thetransmit beamformer 304 causes transmit signals to be provided to eachelement of the two-dimensional transducer element array 402 at abeamforming delay, T_(BF), after the synchronization pulse via, forexample, connection 426, variable gain amplifier 422, connection 424,T/R switch 418, and connection 416. The transmit beamforming delays,T_(BF), are generally different for every element of the two-dimensionaltransducer element array 402 and may be calculated as described below.The transmitted acoustic energy propagates to the target in a time,T_(p), given by $\begin{matrix}{{T_{p} = {\frac{1}{v_{b}}\sqrt{\left( {x - x_{0}} \right)^{2} + \left( {y - y_{0}} \right)^{2} + \left( {z - z_{0}} \right)^{2}}}},} & {{Eq}.\quad 1}\end{matrix}$

[0068] where, v_(b), is the acoustic propagation velocity in the body,the two-dimensional array element is at coordinates (x₀, y₀, z₀), andthe target 1002 is at coordinates (x, y, z). The total time, T, from thesynchronization pulse to the arrival of the transmitted acoustic energyat the target 1002 may be calculated as follows:

T=T _(BF) +T _(p).  Eq. 2

[0069] To focus the acoustic energy at the target 1002, the transmitbeamforming delays, T_(BF), must be chosen so that the total delays, T,are the same for every element, thus causing the ultrasonic energy fromall of the two-dimensional array elements to arrive at the target 1002simultaneously. Any set of transmit beamforming delays, T_(BF), thatsatisfies the condition that all of the total times, T, from thesynchronization pulse to the arrival of the transmitted acoustic energyfrom the individual elements to the target 1002 are the same issufficient. It is clear from the above discussion that to achieve afocus at target 1002, the differences in the transmit beamformingdelays, T_(BF), are completely specified by the geometry.

[0070] On the receive cycle, each element of the two-dimensionaltransducer element array 402 receives the acoustic energy reflected fromthe target 1002 after a propagation delay, T_(p), which is the same asthe propagation delay for that element on transmit. To bring the targetinto focus, the receive beamformer 316 delays the received signal fromeach element by a receive beamforming delay, T_(BF), that is the same asthe transmit beamforming delays. As with transmit beamforming, any setof beamforming delays may be used provided the differences inbeamforming delays between any two elements are correct.

[0071] Furthermore, as the time after the synchronization pulseincreases, the acoustic signals arriving at the two-dimensionaltransducer element array 402 are due to reflections from targets atprogressively deeper depths due to the finite propagation speed of theacoustic energy. The receive beamforming delays, T_(BF), may be changedas a function of depth to provide receive focus at various targetdepths. This is referred to as dynamic receive focusing.

[0072] Interposing a protective cover 206 between the two-dimensionaltransducer element array 402 and the body to be imaged 30 modifies thepropagation delays, T_(p), by an amount related to the acousticvelocities of the protective cover 206, the body to be imaged 30, and tothe thickness of the protective cover 206. Specifically, the protectivecover 206 adds a further protective cover delay, T_(p), givenapproximately by the expression: $\begin{matrix}{{T_{c} = {h \times \left( {\frac{1}{v_{c}} - \frac{1}{v_{b}}} \right)}},} & {{Eq}.\quad 3}\end{matrix}$

[0073] where, h is the thickness of the protective cover 206 and v_(c)is the acoustic velocity within the protective cover 206. If theprotective cover 206 is made of an acoustic-matching material, thenv_(c) is approximately equal to v_(b) and the protective cover delay,T_(c), is approximately zero and no changes are required for thebeamforming delays, T_(BF). Furthermore, if the thickness, h, of theprotective cover 206 is the same for all elements, then the protectivecover delay, T_(c), is the same for all transducer elements regardlessof the velocity, v_(c). Since only the differences in beamforming delaysare important, it is readily seen that a protective cover 206 of uniformthickness does not change the required beamforming delays, T_(BF).

[0074] If, however, the thickness, h, is not uniform over thetwo-dimensional transducer element array 402 and the velocity, v_(c), isdifferent from the velocity, v_(b), then the delay, T_(c), will bedifferent for every element as illustrated in FIG. 12. This will causedistortion of the wave fronts emerging from the protective cover 206during the transmit cycle and entering the protective cover 206 duringthe receive cycle, resulting in loss of focus and blurring of the image.Delays used in the transmit beamformer 304 and the receive beamformer316 can be changed from the nominal values obtained from the distancecalculation so as to cancel the delay variations caused by thenon-uniform thickness of the protective cover 206, thus maintainingfocus and image quality. Stated another way, the new beamformer delaysare equal to the beamformer delays minus the protective cover delays or

T _(new) =T _(BF) −T _(c)  Eq. 4

[0075] In the event that any of the resultant new beamformer delays isnegative, a constant delay can be added to all beamformer channels tomake all delays positive. For example, in FIG. 12, the two-dimensionaltransducer element array 402 is covered by a protective cover 206 havinga non-uniform thickness such that over representative element 1204 thethickness is h₁₂₀₄, and over representative element 1206 the thicknessis h₁₂₀₆. Then the total propagation delay from representative element1204 to the reference plane 1202 is $\begin{matrix}{T_{1204} = {\frac{h_{ref}}{v_{b}} + {h_{1204} \times \left( {\frac{1}{v_{c}} - \frac{1}{v_{b}}} \right)}}} & {{Eq}.\quad 5}\end{matrix}$

[0076] and the total propagation delay from representative element 1206to the reference plane 1202 is $\begin{matrix}{T_{1206} = {\frac{h_{ref}}{v_{b}} + {h_{1206} \times {\left( {\frac{1}{v_{c}} - \frac{1}{v_{b}}} \right).}}}} & {{Eq}.\quad 6}\end{matrix}$

[0077] The procedure described above for calculating beamformer delaysis sufficient to produce a good focus under the most commonlyencountered operating conditions. Implicit in the procedure, however, isthe approximation that the variation across the aperture in the delaygenerated by the protective cover 206 is the same for all steeringangles and focal depths.

[0078] This approximation may not be sufficiently accurate if the beamsteering angle with respect to the transducer face is greater than about45 degrees, or if the active aperture of the transducer 202 is greaterthan the distance to the desired focal point, or if the protective cover206 has a thickness greater than about three wavelengths of the incidentultrasound energy, or if the protective cover 206 has areas in which theradius of curvature is less than about three times the width of the areaover which it occurs.

[0079] An example of this effect is illustrated in FIG. 11. In thisregard, acoustic energy propagates from element 1102 to target 1120along ray 1103, and from element 1112 to target 1120 along ray 1113 andrefracted ray 1114. Acoustic energy may also propagate from element 1102to a second target 1140 along ray 1104 and refracted ray 1105, and fromelement 1112 to the second target 1140 along ray 1115 and refracted ray1116. Although the illustration is not to scale, it will be apparentthat the propagation path lengths through the protective cover 206 andthrough the body to be imaged 30 may be different for the two differenttargets, and that these differences may be different for the twotransducer elements 1102 and 1112. Consequently, the delays, T_(c)through the protective cover 206 become a function not only of theelement position, but also of the target position. Those skilled in theart will appreciate that the illustration presented in FIG. 11 isoffered for simplicity of explanation. Individual transducer elementscannot focus by themselves. A plurality of active transducer elementsacting together under the control of the image processing system 204 canbe made to focus an acoustic beam that traverses the aperture.

[0080] Attention is now returned to the two-dimensional matrix ofcontrollable transducer elements illustrated in FIG. 4. The arrangementshown in FIG. 4 allows a fully sampled, controllable, arbitrary(specified without restraint) two-dimensional delay profile to beapplied to the two-dimensional transducer element array 402. Fullysampled is a term that relates to each transducer element 404, 412 and414 being individually controlled. In preferred embodiments of such anarrangement, each individual transducer element of the two-dimensionaltransducer element array 402 receives some manner of control signal fromthe central controller 310.

[0081] The delay profile of the two-dimensional transducer element arrayaperture is an arbitrary, fully-sampled, controllable function of bothdimensions of the aperture. The delay profile thus may be adjusted tocompensate for any shape of protective cover 206, allowing the shape ofthe protective cover 206 to be specified to provide optimum contact withthe body to be imaged 30, desirable ergonomic qualities, or otherattributes as discussed previously, without degrading the image quality.

[0082] Referring now to FIGS. 5A and 5B, some preferred embodiments oftransducer 202 will be described in greater detail. As depicted in FIG.5A, transducer 202 includes a body 208 and a shaped two-dimensionaltransducer element array 502. As illustrated, the two-dimensionalelement array 502 may comprise a plurality of transducer elements 408,412 (two identified for ease of illustration). Body 208 preferably isconfigured to house one or more of various components required tofacilitate transmission and/or reception of acoustic energy via thetwo-dimensional transducer element array 502. Note that in the presentillustration, the nose 210 and protective cover 206 have been removed toreveal the two-dimensional transducer element array 502. As illustratedin the partial cross-sectional side view of FIG. 5A, the two-dimensionaltransducer element array 502 may be cylindrically shaped. It should beappreciated that a spherically shaped two-dimensional transducer elementarray 502 may be selected for applications where a transducer 202 needsto remain in close contact with various surfaces of the human body asmay be required.

[0083] Furthermore, body 208 may be ergonomically designed to facilitateproper positioning of the transducer 202 for performing an imagingprocedure. The body 208 includes an intermediate portion 504 that isappropriately adapted to be grasped by the hand of an operator.Moreover, the body 208 may be covered by a material that not onlyprotects the transducer electronics but has properties that make thetransducer 202 easy to grasp for a sonographer.

[0084] In the embodiment depicted in FIG. 5A, body 208 contains aprotective cover-mounting portion 506, which preferably flares radiallyin an outward fashion from an intermediate portion 504 adapted to engageprotective cover 206 (not shown). At the proximal end of the transducer202, i.e., the end opposite portion 506, a tapered or necked portion 512is provided. Portion 512 defines an aperture for receiving electricalcordage 520. Cordage 520 is adapted to facilitate electricalcommunication between the transducer 202 and the image processing system204 (not shown).

[0085] Various shapes of protective covers 206 may be utilized toprotect and shield the underlying shaped two-dimensional transducerelement array 502. Preferred shapes will closely cooperate withunderlying two-dimensional transducer element array 502 to provideadequate acoustic coupling. Various considerations, such as the desireto promote good patient contact between the protective cover 206 and thepatient, for image quality and patient comfort, as well as sonographerease of use, for example, may make particular shapes more desirable forparticular ultrasound exams. For instance, in some embodiments, theprotective cover 206 can be physically configured to facilitateconvenient alignment of the transducer 202 with an acoustic window of apatient. In particular, such a protective cover 206 preferablyincorporates curved surfaces extending outwardly from the transducer202. This configuration tends to facilitate convenient positioning ofthe protective cover 206 in relation to an acoustic window, such as anacoustic window defined by adjacent ribs of the patient. Morespecifically, the curved surfaces typically engage the ribs and tend toalign the tissue-engagement surface with the acoustic window. Asdescribed hereinafter, the tissue-engagement surface may be provided invarious configurations.

[0086] As depicted in FIG. 5B, transducer 202 includes a body 208 and ashaped two-dimensional transducer element array 552. As illustrated, thetwo-dimensional array 552 may comprise a plurality of transducerelements 408, 412 (two identified for ease of illustration). Here, as inthe previous figure, the nose 210 and protective cover 206 have beenremoved to reveal the two-dimensional transducer element array 552. Asillustrated in the side view of FIG. 5B, the two-dimensional transducerelement array 552 may be substantially spherically shaped. A sphericallyshaped two-dimensional transducer element array 552 may be selected forapplications where a transducer 202 needs to remain in close contactwith various surfaces of the human body as may be required. It should beappreciated that two-dimensional transducer element arrays 402 havingtoroidal or other curved surfaces (e.g., a saddle surface) are possibleand may use slightly modified beamforming than that associated with theplanar two-dimensional transducer element array 402 illustrated in FIG.4.

[0087] Reference is now directed to FIG. 6, which illustrates animproved method for ultrasound imaging with a two-dimensional transducerelement arrays. In this regard, the method for ultrasound imaging 600starts with step 602, herein designated as “Begin.” The method forultrasound imaging 600 provides a series of time delayed transmitsignals to the shaped transducer as indicated in step 604 toacoustically illuminate a region of interest within a patient's body.The time delays are calculated based on the focusing geometryillustrated in FIG. 10 and any variation in delay profile resulting frompropagation through the protective cover 206.

[0088] In accordance with the improved method for ultrasound imaging600, the generated acoustic energy is propagated through a protectivecover 206 that may be configured to closely mirror the shape of thetwo-dimensional transducer element array 502, 552, etc. as illustratedin step 606. In alternative embodiments, the two-dimensional transducerelement array may be substantially planar with a superposed protectivecover 206 of non-uniform thickness.

[0089] As previously described, the shape of the two-dimensionaltransducer element array 502, 552, may be selected based on a number offactors including patient comfort, sonographer ergonomics, the availableacoustic window of the patient, as well as a host of other factors.

[0090] Next, in step 608, the received ultrasound echoes are capturedand processed preferably by the same two-dimensional transducer 202 usedto perform the transmit function described in steps 604 and 606. Oncethe received ultrasound echoes have been converted in to a voltagewaveform by the transducer 202, the received echoes may be time delayedto focus the ultrasound imaging system 204 as to display desired patientstructures as indicated in step 610. As in step 604, the time delays arecalculated on the basis of the focusing geometry illustrated in FIG. 10and any variation in delay profile resulting from propagation throughthe protective cover 206. It should be appreciated that method steps 604through 610 may be repeated as desired to perform a diagnosticultrasound examination. Any of a number of sonographer generated inputsmay be used to terminate the method for ultrasound imaging 600 asindicated by step 612, herein labeled, “End.”

[0091] As depicted in FIG. 7A, transducer 700 includes a body 702 and aprotective cover 706. The protective cover 706 incorporates a generallyspherical tissue-engagement surface 712, e.g., the tissue-engagementsurface generally is formed as a portion of a sphere.

[0092] As shown in FIG. 7A, the tissue-engagement surface 712 or outersurface of the protective cover 206 is shaped to provide comfort to bothpatient and sonographer. It should be appreciated that the particularshape selected may depend on the type of exam, the size of the patient'sanatomy, and/or other factors. So configured, this embodiment is capableof transmitting acoustic energy from the transducer 702 and propagatingthat energy along a path that is generally coextensive with, or else atsome angle to a longitudinal axis 716 of the transducer 702. Preferably,a length X₇ of the tissue-engagement surface 712 is contacted so as toprovide an appropriate cross-sectional area of engagement with a body 30so that an adequate amount of acoustic energy can be propagated from thetransducer 702 to the body 30.

[0093] As shown in FIG. 7B, the compound geometric structure of avariation of the embodiment depicted in FIG. 7A is described. Morespecifically, as shown in FIG. 7B, the protective cover 706 includes atissue-engagement surface 712 that primarily is defined by a radius R₁(in plan view). The surface defined by radius of curvature R₁transitions at each of its ends to surfaces defined by radii ofcurvature R₂. Preferably, radii R₂ are defined by lengths that permitboth good acoustic coupling with the patient, while maintaining a highlevel of comfort. While radii R₂ are depicted as only slightly shorterthan the length of radius R₁ there are a host of possible relationshipsincluding the substantially spherical tissue-engagement surface formedby the external surface of the protective cover 706 as presented in FIG.7A.

[0094]FIG. 8A illustrates an alternative embodiment of a transducer 800.The transducer 800 has a body 802 and a protective cover 806. Protectivecover 806 is configured as an acoustically non-focusing protective cover806 that substantially follows the shape of an underlyingtwo-dimensional transducer element array (not shown). Preferably,protective cover 806 has a shaped tissue-engagement surface 812 thatresembles a portion of a cylinder.

[0095] As illustrated in FIG. 8A, the transducer 802 may be configuredto form a tissue engagement surface 812 having a width X₈ selected tofacilitate propagation of acoustic energy. However, as illustrated, thewidth also may be selected to exploit an appropriately selected acousticwindow. More specifically, if protective cover 806 is to be utilizedduring a thoracic acoustic-imaging procedure, for example, width X₈ maybe selected so as to attempt to improve transducer positioning betweenadjacently disposed ribs, e.g., ribs 832 and 834, of the body to beimaged 30. So positioned, efficient propagation of acoustic energy fromthe transducer 802, between the ribs, and deeper into the body may befacilitated. As illustrated in FIG. 8A, the protective cover 806 may besubstantially cylindrical in order to efficiently propagate acousticenergy through the acoustic window formed by the ribs 832 and 834.

[0096] As previously described with regard to the substantiallyspherical embodiment of the transducer 702 in FIG. 7B, when viewed fromthe side (FIG. 8B), a tissue-engagement surface 812 formed on theexternal surface of a protective cover 806 may be defined by a radius ofcurvature R₃. Each end of the tissue-engagement surface transitions maybe defined by a radius of curvature R₄ that is modified in length fromradius R₃. The protective cover 806 illustrated in FIG. 8B reveals acase where R4 is less than R₃ So provided the tissue-engagement surface812 presents a relatively flattened surface over the tissue-engagementarea. Thus, the tissue-engagement surface 812 may be viewed as providinga near optimal propagation medium while, advantageously, attempting toexploit the geometry-limited rib access points, among others.

[0097] The external surfaces of protective covers 706 and 806, expectedto form the tissue-engagement surfaces 712 and 812, generally are curvedand can facilitate aligning of the tissue-engagement surface with anacoustic window. More specifically, when the tissue-engagement surfaceis appropriately sized, the external surfaces of the protective covers706, 806 tend to engage the ribs, e.g., ribs 832 and 834, therebyenabling the tissue-engagement surface to nest between the ribs. Thus,the surfaces tend to align the tissue-engagement surface with theacoustic window. The curved surfaces also can enhance patient comfortduring an imaging procedure, as a non-curved surface may tend to causelocalized discomfort.

[0098] It should be appreciated that the protective cover covers 706(FIG. 7B) and 806 (FIG. 8B) are exemplary only. Some embodiments of theimproved transducer may call for arranging a complex protective coverand two-dimensional transducer element array 502, 522 that varies overthe X, Y, and Z dimensions (see FIGS. 5A, 5B, and 10). All suchvariations are contemplated and within the scope of the improvedultrasound transducer.

[0099] Operation

[0100] As depicted in FIG. 9, a preferred embodiment of the transducer202 is shown in operative engagement with a representative acousticwindow. By way of example, the transducer is appropriately positioned atan acoustic window 902 or rib access point of a representative thoracicsection 904 so as to enable acoustic imaging of a heart 906, forexample. As illustrated in FIG. 9, intercostal access points tend to begeometry-limited structures, i.e., the rib access points provide abounded area through which acoustic energy may be propagated (acousticenergy is unable to penetrate bone so as to be useful for imaging). Dueto the shape of protective covers 706, 806 the ability to exploit ribaccess points to provide acoustic imaging of tissues within the bonythorax is potentially increased. Moreover, the material(s) of theprotective cover 706, 806, possessing acoustic impedance much like thatof the body, tends to enhance the amount of acoustic energy propagatedthrough a rib access point. As previously described, the acoustic energycan be electronically focused in both the lateral and elevationaldimensions in both transmit and receive modes to appropriately image thestructures of the heart.

[0101] It should be emphasized that the above-described embodiments ofthe improved ultrasound transducer, particularly, any “preferred”embodiments, are merely possible examples of implementations, merely setforth for a clear understanding of the principles of the transducer.Many variations and modifications may be made to the above-describedembodiment(s) of the improved ultrasound transducer without departingsubstantially from the spirit and principles of the invention.

[0102] For example, although the transducer 202 has been describedherein in relation to an ultrasonic imaging system 204 for use inmedical applications, such as with a patient, such systems may beutilized in various other applications as well. Additionally, varioussurfaces associated with the protective cover 206 have been describedherein as enabling convenient positioning of a transducer 202 relativeto an acoustic window. In other embodiments, one or more of thesesurfaces may be formed as a portion of the transducer body, such as onthe nose of the transducer, to provide similar functionality. All suchmodifications and variations are intended to be included herein withinthe scope of this disclosure and the present invention and protected bythe following claims.

What is claimed is:
 1. An acoustic imaging system, comprising: a transducer including a two-dimensional transducer element matrix array, the transducer having a protective cover configured to mate with a transducer body, the protective cover superposed above the two-dimensional transducer element matrix such that acoustic energy incident at the protective cover is mechanically directed by the protective cover and wherein the transducer element matrix array is encased by the protective cover and the transducer body; and an image processing system coupled to the transducer configured provide a plurality of individualized excitation signals to the plurality of transducer elements over time such that the two-dimensional transducer element matrix array generates and transmits acoustic energy through the protective cover over time such that acoustic energy transmitted through the protective cover is electronically focused.
 2. The acoustic imaging system of claim 1, wherein the protective cover comprises an acoustic material, the acoustic material exhibiting acoustic impedance corresponding to acoustic impedance of a body to be imaged.
 3. The acoustic imaging system of claim 1, wherein at least one of the dimensions of the two-dimensional transducer element matrix array is curved.
 4. The acoustic imaging system of claim 1, wherein the protective cover is constructed with a non-uniform thickness.
 5. The acoustic imaging system of claim 1, wherein the protective cover has an acoustic impedance of between approximately 1.3Mrayl and 1.7MRayl.
 6. The acoustic imaging system of claim 1, wherein the protective cover has a transducer-engagement having a tissue-engagement surface, the transducer-engagement end being configured to engage a transducer body, the tissue engagement surface forming a portion of a substantially cylindrical surface.
 7. The acoustic imaging system of claim 6, wherein the tissue engagement surface forms a portion of a substantially spherical surface.
 8. The acoustic imaging system of claim 1, wherein the transducer body is ergonomically adapted to be grasped by the hand of an operator.
 9. The acoustic imaging system of claim 1, wherein the protective cover has a shape that reduces the probability of a sonographer developing a repetitive motion injury.
 10. The acoustic imaging system of claim 1, wherein the image processing system electronically focuses transmitted acoustic energy at a target by compensating for the non-uniform acoustic delays caused by the protective cover.
 11. The acoustic imaging system of claim 10, wherein the electronic compensation is a function of the position of the target point.
 12. The acoustic imaging system of claim 1, wherein the image processing system receives a plurality of individualized receive mode signals from a plurality of transducer elements, the receive mode signals representative of the incident acoustic energy at a plurality of the transducer elements of the two-dimensional transducer element matrix array that traverses the protective cover.
 13. The acoustic imaging system of claim 12, wherein the image processing system electronically focuses the acoustic energy received through the protective cover.
 14. The acoustic imaging system of claim 13, wherein electronic focusing comprises compensating for the non-uniform acoustic delays caused by the protective cover.
 15. The acoustic imaging system of claim 13, wherein the electronic compensation is a function of the position of the target point.
 16. The acoustic imaging system of claim 15, further comprising: means for accessing an acoustic window of a body to be imaged.
 17. The acoustic imaging system of claim 16, wherein the accessing means comprises placing the transducer between adjacently disposed ribs of the body of a patient.
 18. A method for acoustically imaging a patient, comprising the steps of: providing a transducer having a two-dimensional transducer element matrix array, the transducer having a protective cover configured to mate with a transducer body, the protective cover superposed above the two-dimensional transducer element matrix such that acoustic energy transmitted from the protective cover and into the body is mechanically directed by the protective cover, wherein the two-dimensional transducer element matrix array and the protective cover are shaped to reduce patient discomfort; generating a plurality of time delayed transmit signals to separately control individual transducer elements of the two-dimensional transducer element matrix array to electronically focus acoustic transmit waves that traverse the protective cover; and receiving a plurality of time delayed response echoes at the separately controllable individual transducer elements of the two-dimensional transducer element matrix array to electronically focus acoustic receive echoes that traverse the protective cover.
 19. The method of claim 18, further comprising the step of processing the reflected acoustic echoes to generate an image.
 20. The method of claim 18, further comprises the steps of accessing an acoustic window of a patient; and transmitting acoustic energy through the protective cover and into the patient via the acoustic window.
 21. The method of claim 18, wherein the steps of generating and receiving further comprise: electronically focusing the acoustic energy in an elevation dimension; and electronically focusing the acoustic energy in a lateral dimension.
 22. The method of claim 20, wherein the step of accessing an acoustic window comprises an acoustic window formed between adjacently disposed ribs of the patient. 